The primary objective of this study is to introduce a novel concept, namely spinal geometric compensation, to the realm of spine biomechanics. Mechanical low back pain has long been associated with lumbar segmental instability, wherein the degree of segmental mobility profoundly impacts the curvature and range of motion (ROM) of the thoracolumbar spine. Consequently, this research endeavors to shed light on the intricate geometric compensatory mechanisms employed by the spine to uphold congruency. A validated and representative finite element model of the thoracolumbar spine was developed, to comprehensively evaluate the influence of passive tissues such as the lumbar intervertebral disc, iliolumbar ligament, the rib cage, and the lower lumbar facet cartilage-capsular ligament on lumbar segmental instability and spinal geometric compensation during flexion and extension rotations.